iOS应用程序的状态

• iOS应用程序一共有五种状态：

• Not Running 程序还没运行
• Inactive 程序运行在foreground但没有接收事件
• Active 程序运行在foreground接收事件
• Background 程序运行在background正在执行代码
• Suspended 程序运行在background没有执行代码

• iOS应用程序状态变化会回调APPDelegate中的方法，但不是每一种状态变化都会有对应的方法(上图的红框的两个变化就没有对应的方法)
  • application:didFinishLaunchingWithOptions: Not Running -> Inactive
  • applicationDidBecomeActive: Inactive -> Active
  • applicationWillResignActive: Active -> Inactive
  • applicationDidEnterBackground: Background -> Suspended
  • applicationWillEnterForeground: Background -> Inactive
  • applicationWillTerminate: Suspended -> Not Running

• 常见的应用状态变化场景

  • 程序第一次启动（或者被杀掉以后启动）：
    Not Running -> Inactive -> Active
  • 点击Home键（没有在Inof.plist中设置Application does not run in background）：
    Active -> Inactive -> Background -> Suspended
  • 点击Home键（在Inof.plist中设置Application does not run in background为YES，应用不能运行在后台，进入后台后会立即进入Not Running）：
    Active -> Inactive -> Background -> Suspended -> Not Running
  • 挂起重新运行
    Suspended -> Background -> Inactive -> Active
  • 内存清除（杀掉应用或删除应用）
    Suspended -> Not Running
  • 应用之间的切换
    Active -> Inactive
    Inactive -> Active
  • 点击Home键（在Inof.plist中设置Application does not run in background为YES，应用不能运行在后台，进入后台后会立即进入Not Running）：
    Active -> Inactive -> Background -> Suspended -> Not Running

• 可通过在APPDelegate的回调方法中打印数据，来查看应用状态变化

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

    print("`application:didFinishLaunchingWithOptions:`  Not Running -> Inactive")
    return true
}

func applicationWillResignActive(_ application: UIApplication) {

    print("`applicationWillResignActive:`  Active -> Inactive")
}

func applicationDidEnterBackground(_ application: UIApplication) {
    
    print("`applicationDidEnterBackground:`  Background -> Suspended")
}

func applicationWillEnterForeground(_ application: UIApplication) {
    
    print("`applicationWillEnterForeground:` Background -> Inactive")
}

func applicationDidBecomeActive(_ application: UIApplication) {

    print("`applicationDidBecomeActive:`  Inactive -> Active")
}

func applicationWillTerminate(_ application: UIApplication) {

    print("`applicationWillTerminate:`  Suspended -> Not Running")
}